This invention relates to a method of and an apparatus for tightening high-strength steel bolts in a pretensioned state, and more particularly to an improved method of and apparatus for tightening high-strength steel bolts with greater accuracy and with greater efficiency.
In conventional steel construction using high-strength steel bolts for joining the structural members, the widely-used process consists of tightening high-strength steel bolts in the manner in which ordinary bolts other than the high-strength steel bolts are tightened by simply turning the mated nuts. However, high-strength steel bolts are subjected to an amount of axial force greater than the ordinary bolts, and the frictional resistance produced by the contact of the mated nut against the face of an object to be fastened or the face of a washer increases with the greater axial force to which the bolt is subjected, so that the greater torque is required to tighten the bolt. This results in the tightening work becoming extremely difficult, and it is impossible exactly to determine the desired axial force applied to the bolt.
In order to improve the known tightening method above mentioned and to overcome the problems thereof, it is conceivable that high-strength steel bolts may be tightened by using the pretensioning method, and there is a device practically employed for this purpose. In the pretensioning method, the known device has an extremely limited use, and therefore is only applicable to fastening prestressed concrete steel bars or bolts having a relatively long shank such as ordinary anchor bolts. However, there are not known hereto devices which can practically be used for tightening bolts having a relatively short shank, such as the bolts employed for joining ordinary structural members of bridges, buildings and the like, for example.
In the pretensioning method using the short-shank bolts, the mated nuts are tightened with the bolts in a pretensioned state. However, a nuts are subjected to the compressive force as soon as the pretensioned bolts are freed from the tensile force. As a result, the bolts are relaxed to greater degrees from the tensile force, which makes it difficult to obtain the desired axial force.